Crude Oil Distillation. Chapter 4

Transcription

1 Crude Oil Distillation Chapter 4

2 Gases Gas Sat Gas Plant Polymerization LPG Sulfur Plant Sulfur Alkyl Feed Alkylation Butanes Fuel Gas LPG Gas Separation & Stabilizer Light Naphtha Heavy Naphtha Isomerization Naphtha Hydrotreating Naphtha Reforming Isomerate Polymerization Naphtha Alkylate Reformate Naphtha Aviation Gasoline Automotive Gasoline Solvents Atmospheric Distillation Crude Oil Desalter Vacuum Distillation AGO LVGO HVGO Distillate Gas Oil Hydrotreating Kerosene Fluidized Catalytic Cracking Hydrocracking Cat Distillates Cycle Oils Cat Naphtha Fuel Oil Distillate Hydrotreating Treating & Blending Jet Fuels Kerosene Solvents Heating Oils Diesel Residual Fuel Oils DAO Solvent Deasphalting Coker Naphtha SDA Bottoms Naphtha Asphalts Vacuum Residuum Visbreaking Coking Heavy Coker Gas Oil Light Coker Gas Oil Distillates Fuel Oil Bottoms Solvent Dewaxing Lube Oil Waxes Lubricant Greases Waxes Coke 2

3 Atmospheric & Vacuum Distillation in U.S. EIA, Jan. 1, 2017 database, published June

4 Topics Crude Stills Historically the oldest refining process Only the first step in crude oil processing Purpose To recover light materials Fractionate into sharp light fractions Configuration May be as many as three columns in series Crude Stabilizer/Preflash Column Reduce traffic in the Atmospheric Column Atmospheric Column Vacuum Column Reduced pressure to keep blow cracking temperatures Product Yield Curves Cut Point, Overlap, & Tails 4

5 Configuration: Atmospheric Vacuum Preflash

6 Atmospheric & Vacuum Tower Complex 6

7 Atmospheric & Vacuum Tower Complex Modified drawing from: Revamping crude and vacuum units to process bitumen, Sutikno, PTQ,Q

8 Atmospheric Column with Preflash Modification of figure in Increasing distillate production at least capital cost, Musumeci, Stupin, Olson, & Wendler, PTQ, Q

9 Preflash Options Tight Oil Example with no AGO Optimising preflash for light tight oil processing, Lee, PTQ, Q

10 Feed Preheat Train & Desalter Feed Preheat Train Initial heat exchange with streams from within the tower Heat recovery important to distillation economics! o First absorb part of the overhead condensation load o Exchange with one or more of the liquid sides streams, beginning with the top (coldest) side stream Require flexibility o o o Changes in crude slate Temperature at desalter Limits on two phase flow through network Final heating in a direct fired heater Heat enough to vaporize light portions of the crude but temperature kept low to minimize thermal cracking o Inlet typically 550 o F, outlet 600 to 750 o F. o Heavier crudes cannot be heated to the higher temperatures Desalter Temperature carefully selected do not let water vaporize Lighter crudes (> 40 o 250 o F Heavier crudes (< 30 o 300 o F All crudes contain salts (NaCl, MgCl, ) Salt present in the emulsified water Treated in the field with heat & chemicals to break oil water emulsions. Salt can cause damage to equipment o o o Scale in heat exchangers HCl formation can lead to corrosion Metals can poison refinery catalysts Remove salts & dissolved metals & dirt Oil mixed with fresh wash water & demulsifiers. Separation in electrostatic settling drum Wash water up to 10% of crude charge ~ 90% of the water can be recovered Effluent water treated for benzene 10

11 Crude Electrostatic Desalting Drawing by Milton Beychok BFDs from: Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD ROM,

12 Crude Desalting Breaking the crude oil/water emulsion important to minimize downstream problems Performance of additives may be crude specific Picture from: Removing contaminants from crude oil McDaniels & Olowu, PTQ Q1,

13 Direct Fired Heater Ref: Useful tips for fired heater optimisation Bishop & Hamilton, Petroleum Technology Quarterly, Q

14 Atmospheric Distillation Summary Condenser Partial condenser if no Stabilizer Column. Total condenser if Stabilizer Column to remove light ends. but no reboiler. Feed preheat exchanger train All of the heat to drive the column comes from the hot feed. As much as 50% of the incoming crude may be flashed. Overflash o Extra amount of material vaporized to ensure reflux between flash zone & lowest side draw o Typically 2 vol% of feed Pumparounds Move cooling down column. Liquid returned above draw tray Side draws Side strippers Clean up side products Stripping steam Reduce hydrocarbon partial pressure Condensed & removed as a second liquid phase. Conditions set so it doesn t condense within the column can lead to foaming Must be treated as sour water 14

15 Atmospheric Distillation Summary Wash Zone Couple trays between flash zone & gas oil draw. Reflux to wash resins & other heavy materials that may contaminate the products. Condenser Typically 0.5 to 20 psig. Balancing act Low pressures reduce compression on overhead system High pressures decrease vaporization but increase flash zone temperatures & furnace duty; affects yields Pumparounds Reduces overhead condenser load & achieves more uniform tower loadings Provides liquid reflux below liquid draws Side Draws & Strippers Side strippers remove light component tail & return to main column Steam strippers traditional Reboiled strippers reduce associated sour water & may reduce steam usage Trays & Pressure Profile Typically 32 trays in tower 0.1 psi per tray for design & target for operation May find as high as 0.2 psi per tray, but probably flooding! Condenser & accumulator 3 to 10 psi across condenser Liquid static head in accumulator Typically 6 to 16 psi across entire column. 15

16 Vacuum Distillation Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD ROM, 2000 Consider practical conditions for vacuum unit modeling R. Yahyaabadi, Hydrocarbon Processing, March

17 Vacuum Distillation Trays vs. Packing Packing used in vacuum towers instead of trays Lower pressure drops across the tower vapor slides by liquid instead of pushing through the layer on the tray Packing also helps to reduce foaming problems Foaming in fractionation columns M. Pilling, PTQ, Q

18 Vacuum Distillation Summary Column Configuration Vacuum conditions to keep operating temperatures low Large diameter column Very low density gases Condenser only for water vapor Liquid reflux from pumparounds No reboiler Stripping steam may be used Needed for deep cuts (1100 o F) Common problem coking in fired heater & wash zone Fired heater high linear velocities to minimize coke formation Wash zone sufficient wash oil flow to keep the middle of the packed bed wet Feed Atmospheric residuum All vapor comes from the heated feed Under vacuum (0.4 psi) Separate higher boiling materials at lower temperatures Products Minimize thermal cracking May have multiple gas oils Usually recombined downstream to FCCU after hydrotreating Vacuum resid Blended asphalt, heavy fuel oil Further processing thermal, solvent o Depends on products & types of crude 18

19 Vacuum Distillation Summary Dry System 1050 o F+ cut temperature & no stripping steam Smaller tower diameters Reduced sour water production Pressure profile Flash zone: mmhg abs & 750 to 770 o F. Top of tower: 10 mmhg abs Steam Ejectors & Vacuum Pumps Vacuum maintained on tower overhead Steam systems considered more reliable Waste steam is sour & must be treated Combinations systems Last steam stage replaced with a vacuum pump Deep Cut System 1100 o F+ cut temperature & stripping steam Steam reduces hydrocarbon partial pressures Pressure profile Flash zone: 30 mmhg abs HC partial pressure mmhg abs Top of tower: 15 mmhg abs Drawing from 19

20 Example Crude Preheat Trains Ref: Improve energy efficiency via Heat Integration Rossiter, Chemical Engineering Progress, December

21 Composite Curve for Preheat Train Compare amount of heat available & at what temperatures Goal is to shift the hot & cold composite curves as close as possible Pinch technology This will reduce the amount of excess heat to be thrown away to the environment This will also reduce the amount of fresh heat added to the system Ref: Energy savings in preheat trains with preflash Bealing, Gomez Prado, & Sheldon, PTQ, Q

22 Example Existing Preheat Train Ref: Energy savings in preheat trains with preflash Bealing, Gomez Prado, & Sheldon, PTQ, Q

23 Example Improved Preheat Train Ref: Energy savings in preheat trains with preflash Bealing, Gomez Prado, & Sheldon, PTQ, Q

24 Product Yield Curves

25 Typical Cut Point Definitions Cut TBP IBP ( o F) TBP EP ( o F) Light Naphtha (LSR Gasoline) 80 to to 220 Heavy Naphtha 180 to to 380 Middle Distillate (Kerosene) AGO (Atm Gas Oil) LVGO (Light Vac Gas Oil) HVGO (Heavy Vac Gas Oil) Vacuum Resid 950 to to to to to

26 Product Yield Curves Cut Point, Overlap, & Tails Industrial distillation columns do not provide perfectly sharp separations Initial calculations using crude oil assays assume that all materials at a certain boiling point goes to one product or another Imperfect separations result in light ends & heavy ends tails in adjacent products Presence of tails complicate the definition of cut point Analysis Scale distillation curves to represent the volume removed Cut point temperature represents the feed s TBP corresponding the cumulative volume removed Tail represents the light fraction s amount above the cut point & the heavy fraction s amount below the cut point Ref: R.N. Watkins, Petroleum Refinery Distillation, 2 nd ed.,

27 Example Atmospheric Tower Products 27

28 Example Atmospheric Tower Products 28

29 Example Scale to Fraction of Crude Charge 29

30 Scale to Fraction of Crude Charge 30

31 Cut Points & Overlaps for Example 31

32 Boiling Point Ranges for Example 32

33 Summary 33

34 Summary Reported refinery capacity tied to charge to crude distillation complex Increase capacity with Pre flash column Complex column configurations No reboilers, heat from feed furnaces Reuse heat via heat exchange between feed & internal column streams Side draws, pumparounds, side strippers Pumparounds ensure proper liquid reflux within the column Stripping steam 3 phase condensers Condensed water will have hydrocarbons & dissolved acid gases Pre heat train recycles heat Products & internal streams heat the feed Feed cools the internal streams & products Vacuum column to increase the effective cut points Vacuum columns large diameter to keep vapor velocities low Vacuum gas oils recombined only separated for operating considerations Pressure drops are important, especially in the vacuum column Steam stripping aids in separation without cracking Metals are undesirable. Can remove some metals via desalters. 34

35 Supplemental Slides

36 Crude Distillation Unit Costs Atmospheric column includes Side cuts with strippers All battery limits process facilities Heat exchange to cool products to ambient temperature Central control system Petroleum Refining Technology & Economics, 5 th ed. Gary, Handwerk, & Kaiser CRC Press,

37 Crude Distillation Unit Costs Vacuum column includes Facilities for single vacuum gas oil 3 stage vacuum jet system at mmhg Heat exchange to cool VGO to ambient temperature Petroleum Refining Technology & Economics, 5 th ed. Gary, Handwerk, & Kaiser CRC Press,

38 Crude Distillation Technologies Provider Foster Wheeler Shell Global Solutions TECHNIP Uhde GmbH Features Complex of atmospheric & vacuum distillation for initial separation of crude oil. May include pre flash column. Vacuum distillation 38

39 Typical Distillation Column Top of column condenser to remove heat Provides liquid reflux through top of column Partial condenser may have vapor but no liquid distillate product Coldest temperature cooling media must be even colder Lowest pressure Top section strips heavy components from the rising vapors Feed Vapor, liquid, or intermediate quality Introduced in vapor space between trays Internals Trays to contact rising vapors with falling liquids Pressure drop across trays overcome static head of liquid on tray, Bottom of column reboiler to add heat Provides vapor traffic in bottom of column Highest temperature heating media must be even hotter Highest pressure Bottom section strips light components from the falling liquid Drawing by Henry Padleckas & modifed by Milton Beychok: 39

40 Fractionation Columns & Trays Drawings by Henry Padleckas 40

41 Fractionation Tray Types 41

42 Trays & Packing details/tower Internals html 42

43 Typical Overall Efficiencies Column Service Typical No. of Actual Trays Typical Overall Efficiency Typical No. of Theoretical Trays Simple Absorber/Stripper Steam Side Stripper Reboiled Side Stripper Reboiled Absorber Deethanizer Depropanizer Debutanizer Alky DeiC4 (reflux) Alky DeiC4 (no reflux) Naphtha Splitter C2 Splitter C3 Splitter C4 Splitter Amine Contactor Amine Stripper Crude Distillation Stripping Zone Flash Zone 1 st draw st Draw 2 nd Draw nd Draw 3 rd Draw Top Draw Reflux Vacuum Column (G.O. Operation) Stripping Flash Zone HGO Draw HGO Section LGO Section FCC Main Fractionator Quench Zone Quench HGO Draw HGO LCGO LCGO Top Viscosity cp Maxwell Ave Viscosity of liquid on plates Drickamer & Bradford in Ludwig Molal Ave Viscosity of Feed Refinery Process Modeling Gerald Kaes, Athens Printing Company, 2000, pg. 32 Rules of Thumb for Chemical Engineers, 4th ed. Carl Branan, Gulf Professional Publishing, 2005 Engineering Data Book, 12th ed. Gas Processors Association,

44 Vacuum Tower Transfer Lines Mass transfer effects in the transfer line complicate the effects at the bottom of the Vacuum Tower Myth of high cutpoint in dry vacuum units, S. Golden, T. Barletta, & S. White, PTQ, Q